This invention relates to the field of micromechanical systems, particularly to electrostatically deflectable micromechanical systems, more particularly to electrostatically deflectable micromirror devices.
Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device. Yet another recent architecture, the spring-ring micromirror device, promises improved operating performance. The spring-ring architecture and other recent architectures have enabled micromirror cells to shrink from a 16 xcexcm mirror on 17 xcexcm centers to smaller and smaller dimensions. As the micromirror cells shrink, it becomes much more difficult to reliably control the micromirror over the range of device variances encountered in production and over the lifetime of the device. What is needed is an improvement to the electrostatic efficiency of the basic electrostatic deflection mechanisms of digital micromirrors and other micromechanical devices.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for improved electrostatic efficiency of micromirror and other micromechanical devices. One embodiment of the claimed invention provides a micromechanical device having improved electrostatic efficiency. The device comprises a first member forming one half of an air-gap capacitor, and a second member forming one half of an air gap capacitor. The second member is spaced apart from the first member by an air gap and shaped such that the air gap is smaller in a first region than in a second region.
A second embodiment of the disclosed invention provides a micromirror device. The micromirror devices comprises: a substrate, an address electrode supported by the substrate, a deflectable member supported by the substrate and spaced apart from the address electrode by a gap. The deflectable member is operable to rotate around a hinge axis. The gap between the address electrode and the deflectable member is narrower over a first region near the hinge axis and wider over a second region farther from the hinge axis. The variance in the width of the gap can be formed by using a beveled or stair stepped electrode. According to one embodiment, the stair stepped electrode is formed by encapsulating an oxide layer between two metal layers making up the electrode.
According to yet another embodiment of the disclosed invention, an image projection system is provided. The image projection system comprises: a light source for providing a beam of light along a first path, micromirror device on the first path, and a controller for providing image data to the micromirror device.
The micromirror device comprises: a substrate, an address electrode supported by the substrate; and a deflectable member supported by the substrate and spaced apart from the address electrode by a gap. The deflectable member rotates around a hinge axis to deflect into the gap. The gap is narrower over a first region close to the hinge axis and wider over a second region farther from the hinge axis.
Image data from the controller directs the micromirror device to rotate the deflectable member to either a first position wherein light incident the deflectable member is directed along a second path to an image plane, or to rotate the deflectable member to a second position wherein light incident the deflectable member is directed along a third path away from the image plane.
The disclosed micromirror device has increased efficiency since the narrowed gap provides a stronger electrostatic attractive force between the address electrode and the deflectable member when the deflectable member is in an undeflected state, while avoiding contact between the address electrode and the deflectable member when the deflectable member is in a deflected state. Contact between the two could destroy the device and would limit the deflection angle leading to an image with reduced contrast.